This invention relates generally to satellite communications systems and, in particular, to RF transmission and power control techniques for use in a satellite communications system.
Satellite telephone systems for fixed and mobile communications are emerging as a new global business. These systems utilize many individual circuits routed through one satellite or a constellation of many satellites to effect communications. The value of the satellite telephone system is that it provides ubiquitous coverage of large areas of the earth without the construction of many small terrestrial cells. Since the allocation of frequencies for satellite services, a number of proposals have been advanced for the deployment of satellite communications systems. In general, these proposals have involved either a Time Division Multiple Access (TDMA) technique or a Code Division Multiple Access (CDMA) technique.
The communications link availability for these services are a critical factor. In high frequency bands above 3 GHz, and especially above 10 GHz, it is important to avoid a large amount of margin in the signal strength in order to avoid oversizing the satellite design. Further it is important for some systems, such as CDMA systems, to maintain the signal at a fixed level as it arrives at the satellite. An important consideration then is the method selected for compensating for rain attenuation in the frequency bands above 3 GHz, and for other types of signal path impairments as well.
Referring to FIG. 1A, there is shown an arrangement for communication to several satellites 2 for a typical Teleport or Mobile Satellite Service (MSS) site 1. The site 1, also referred to as a gateway, is a typical multi-satellite teleport having antennas 1a in contact with several satellites 2, each of which is connected to one or more gateways or served entities. The multiple antennas la connected to a single gateway 1 may provide multiple satellite coverage, such as is proposed for Low Earth Orbit (LEO) Mobile Satellite Service (MSS) or Fixed Satellite Service (FSS).
FIG. 1B illustrates the same site 1 with the addition of a spatial diversity site 1b separated from the primary site by a distance D. In this approach the spatial diversity of antenna sites provides alternative signaling paths to a single one of the satellites 2. In the bands above 10 GHz, in order to maintain high levels of availability, it is conventional practice to place the redundant or diversity site 1b some 35 km to 100 km away from the primary site 1. By thus separating the sites the communications can be switched to the diversity site 1b when atmospheric attenuation, due, for example, to a presence of a rain cell near the primary site 1, exceeds a certain value at the primary site.
Alternatively, a second approach eliminates the diversity site 1b by placing the primary site 1 in a region, such as a desert, where rain is infrequent.
Neither of these approaches is satisfactory, and both incur large incremental costs. In the first approach there must be a duplication of hardware, real estate, and possibly personnel if the sites are manned. Also, some mechanism must be installed for linking the primary site 1 to the diversity site 1b (e.g., underground cables, microwave towers, etc.). The second approach requires the construction, provisioning, and maintenance of the site in a location which is either inconvenient or is not economically attractive (e.g., the site is too far from the PSTN connection, requiring long distance backhauls).
It can be realized that the communications capability is enhanced by selecting the communications path or paths with the lowest attenuation, or by combining lowest attenuation paths, while avoiding paths which are heavily attenuated. This approach maximizes communications signal strengths and reduces the amount of signal strength margin required. That is, instead of transmitting at a power level that is required to compensate for the heavily attenuated paths, and thus consuming a considerable amount of satellite power, a better approach is to avoid the heavily attenuated path or paths in favor of the less attenuated path or paths. In order to accomplish this technique it is necessary to make decisions based on observed amounts of path attenuation.
In previous systems known to the inventor such decisions were made based on received signal strength, at the gateway, of a signal transmitted from some source, generally located on the satellite or passed through the satellite. However, a disadvantage of this approach is that the attenuation information is only instantaneously known, thereby making impossible an ability to perform short term or long term planning of link allocation and power budgets.
Reference in regard to various satellite power control techniques can be had to the following U.S. Pat. No. 4,991,199, Saam, xe2x80x9cUplink Power Control Mechanism For Maintaining Constant Output Power From Satellite Transponderxe2x80x9d; U.S. Pat. No. 4,752,967, Bustamante et al., xe2x80x9cPower Control System For Satellite Communicationsxe2x80x9d; U.S. Pat. No. 5,339,330, Mallinckrodt, xe2x80x9cIntegrated Cellular Communications Systemxe2x80x9d; U.S. Pat. No. 4,752,925, Thompson et al., xe2x80x9cTwo-Hop Collocated Satellite Communications. Systemxe2x80x9d; U.S. Pat. No. 5,126,748, Ames et al., xe2x80x9cDual Satellite Navigation System And Methodxe2x80x9d; U.S. Pat. No. 5,109,390, Gilhousen et al., xe2x80x9cDiversity Receiver In A CDMA Cellular Telephone Systemxe2x80x9d; and U.S. Pat. No. 5,138,631, Taylor, xe2x80x9cSatellite Communication Networkxe2x80x9d.
Reference can also be had to the improved power management technique disclosed in commonly assigned and allowed U.S. patent application Ser. No. 08/467,209, filing date: Jun. 6, 1995, entitled xe2x80x9cClosed Loop Power Control For Low Earth Obrbit Satellite Communications Systemxe2x80x9d, by Robert A. Wiedeman and Michael J. Sites.
Reference may also be had, by example, to xe2x80x9cSatellite Communications System Engineeringxe2x80x9d, 2nd Edition, W. Pritchard et al., Prentice Hall, 1993, pages 273-294, for a discussion of various noise temperatures, propagation factors, and the use of a rain attenuation model in RF link design.
It is a first object of this invention to provide an improved satellite communication system and method for accurately determining the presence and amount of attenuation due to rain and other weather-related events.
It is a second object of this invention to provide an improved satellite communication system and method that employs a direct measurement of severe path attenuation potential, without requiring a signal source either transmitted from a satellite or passed through the satellite.
An advantage provided by the teaching of this invention is an ability to operate a satellite communications system so as to conserve system power.
A further advantage provided by the teaching of this invention is an ability to operate a satellite communications system so as to assign and allocate resources in accordance with a current model of RF signal path impairments between gateways, satellites, and user terminals, on a local or global scale.
The foregoing and other problems are overcome and the objects and advantages are realized by methods and apparatus in accordance with embodiments of this invention.
This invention pertains particularly to satellite communications systems using GSO or NGSO satellites. This invention employs data derived from signals of various types to derive a model of atmospheric-related attenuation-inducing events so as to plan system resource allocation to minimize an amount of power required to close communication links between user terminals and the satellites. Disclosed are methods for modelling gateway to satellite links, as well as methods for modeling the user terminal to satellite links.
This invention avoids the necessity to employ diversity antenna sites, but does not preclude the use of such sites, by modeling atmospheric disturbances in real time by measurement, by predicting the xe2x80x9cbestxe2x80x9d path to use, and by directing the antennas at the site to utilize a least faded path and/or to select alternate paths to avoid significant fading. The use of the invention at a diversity antenna site improves system performance.
This invention overcomes the problems inherent in the prior art by providing a direct measurement of severe path attenuation potential, without requiring a signal source either transmitted from the satellite or passed through the satellite. The use of the teaching of this invention enables preplanning of the allocation of satellites and satellite resources by calculating a potential for signal path interference based on an external measurement of where rain attenuation may be located, and the real-time or approximately real-time tracking of rain cell activity within storm cells.
A method is disclosed, in accordance with this invention, for operating a satellite communications system of a type that includes a plurality of ground stations, a plurality of satellites, and a plurality of user terminals. The method includes the steps of, at a plurality of the gateways, generating a model of atmospheric-related attenuation-inducing structures and/or other propagation factors located within a coverage area of each of the gateways; transmitting data indicative of the generated model from each of the plurality of gateways to a gateway controller; and generating gateway commands at the gateway controller for preemptively allocating satellite communications system resources at least in accordance with the received data.